I. Field
The present invention relates generally to communication, and more specifically to techniques for controlling the transmit power of a data transmission on multiple transport channels in a wireless communication system.
II. Background
In a wireless communication system, a user with a terminal (e.g., a cellular phone) communicates with another user via transmissions on the downlink and uplink with one or more base stations. The downlink (i.e., forward link) refers to the communication link from the base station to the terminal, and the uplink (i.e., reverse link) refers to the communication link from the terminal to the base station. In a Code Division Multiple Access (CDMA) system, a base station can transmit data to multiple terminals simultaneously. The total transmit power available at a base station thus determines the downlink capacity of the base station. A portion of the total available transmit power is allocated to each terminal such that the aggregate transmit power for all active terminals is less than or equal to the total available transmit power.
To maximize downlink capacity, a power control mechanism is typically used for each terminal. The power control mechanism is normally implemented with two power control loops, which are commonly referred to as an “inner” loop and an “outer” loop. The inner loop adjusts the transmit power used for the terminal such that the received signal quality (SIR) for a downlink transmission, as measured at the terminal, is maintained at an SIR target. The received signal quality may be quantified by a signal-to-noise-plus-interference ratio or some other quantity. The outer loop adjusts the SIR target to achieve the desired level of performance, which may be quantified by a block error rate (BLER) target or some other performance measurement. By minimizing the amount of transmit power used for the terminal while maintaining the BLER target, increased system capacity and reduced delays in serving users can be achieved. When BLER is too low, the terminal consumes too much system capacity. Conversely, when the BLER is too high, the terminal suffers from unsatisfactory service to the user.
A Wideband CDMA (W-CDMA) system supports data transmission on one or more “transport” channels to each terminal. A transport channel may be viewed as a data/message bearer. Each transport channel is associated with one or more transport formats, and each transport format specifies various processing parameters for that transport channel. A BLER target may also be specified for each transport channel. Each transport channel may require a different SIR target, which is dependent on both the BLER target and the transport formats selected for that transport channel.
In W-CDMA, one or more transport channels are multiplexed onto a “physical” channel. The transmit power for the physical channel (and not the individual transport channels) is adjusted through power control. Power control for a single physical channel carrying multiple transport channels with different SIR targets is challenging.
In one conventional design, a separate outer loop is maintained for each transport channel. The outer loop for each transport channel adjusts the SIR target for that transport channel based on the status of data blocks received on that transport channel. In particular, the SIR target for a given transport channel may be decreased by a small amount if a good data block is received on the transport channel, increased by a large amount if a bad data block is received, and maintained at the same level if no data blocks are received. A final SIR target for the physical channel is then set to the highest SIR target among the SIR targets for all of the transport channels carried by the physical channel. The inner loop then adjusts the transmit power for the physical channel to achieve the final SIR target. The use of the highest SIR target among all transport channels as the final SIR target for the physical channel ensures that the BLER target or better is achieved for all transport channels carried by the physical channel.
The above design works well if all transport channels carried by the physical channel are active at all times. However, if the transport channel with the highest SIR target is inactive or intermittently active, then the SIR target for this transport channel will dominate the power control for the physical channel. This is because the SIR target for this transport channel will be maintained at a high level due to little or no activity on the transport channel. Consequently, the final SIR target will be continually set to the high SIR target for this transport channel. Excess transmit power is then used for the physical channel and system capacity is wasted.
There is therefore a need in the art for techniques to control the transmit power for multiple transport channels multiplexed together.